[Example Request] - HPO with Spark and R algo

**Describe the use case example you want to see**
Hyperparameter optimization using spark and custom R algorithm

**Describe which SageMaker services are involved**
This would likely require a custom Docker container as well as spark.

Note, I see the tune_r_bring_your_own.ipynb as well as the pyspark_mnist_custom_estimator.ipynb scripts and would likely model an example off of these. However, I am unsure if PySpark would accept an image written in R.

Any insight into how one could approach this would be greatly appreciated!

Start by comparing tune_r_bring_your_own.ipynb and pyspark_mnist_custom_estimator.ipynb, the only entry points named in the issue. Clarify whether the requested Spark, custom R algorithm, and hyperparameter optimization combination is supported, then define an end-to-end example and its required container setup as the completion criteria.

Written by the indexing model from the issue text.